Device and method for the calibration and control of thermal detectors

ABSTRACT

The invention relates to a device and method for communicating with a thermal detector module and an appropriate thermal detector for a use with such a control device. The device for communicating with a thermal detector module comprises a means for providing a collimated beam of thermal infrared radiation and means for modulating the intensity of the collimated beam of thermal infrared radiation. The intensity modulated thermal infrared beam, when directed on to the thermal detector module from a position within the module&#39;s field of view, is suitable for triggering a response from the thermal detector module. The intensity modulated thermal infrared beam may also be encoded with a digital signal to control the function of the thermal detector.

This application is the US national phase of international applicationPCT/GB03/00429, filed 03 Feb. 2003, which designated the US and claimspriority to GB Application No. 0202472.7 filed 02 Feb. 2002. The entirecontents of these applications are incorporated herein by reference.

This invention relates to thermal detectors and in particular to adevice and method for calibrating and/or communicating therewith.

All objects emit radiation with an intensity and wavelength distributionthat is determined by their surface temperature and surface finish. Forobjects (such as bodies) around room temperature the energy peaks at 10μm, which is in the long wavelength infra-red (LWIR) transmission bandthat runs from 8-14 μm. As this type of radiation is related to thetemperature of an object it is referred to as thermal infraredradiation.

Thermal detectors (sometimes called bolometers) are known radiationdetectors where the detection element absorbs radiation falling on itand, as a result, increases in temperature. This rise in temperature isused to generate a change in the element that can be measuredexternally, giving an output signal proportional to the radiationreceived. These detectors are typically used to detect energy in theLWIR band.

One type of well known, and commercially available, thermal detector isthe single element pyroelectric detector; these detectors are commonlyreferred to as passive infrared (PIR) sensors. PIR sensors are widelyused in many applications, such as intruder alarm and automated lightingsystems, and are designed to provide a response (i.e. to be triggered)by the thermal signature of a moving body or bodies within a certainfield of view. Typically, the sensitivity and field of view of suchsensors is designed, and fixed, for a specific application.

PIR sensors detect transient changes in their thermal surroundings. Thismeans that only changes in the received energy over a relatively shorttime scale trigger the sensor. In this way changes in temperature overlonger time scales, for example those associated with a heat sourcebeing used to heat a room, are ignored whilst transient movements, suchas a human body moving across a room, are sufficient to trigger thesensor. Furthermore, certain PIR sensors employ pulse countingtechniques to further reduce the possibility of false alarms. Thesedevices require two or more transient changes to occur before they aretriggered.

The plastic optical components incorporated within conventional PIRsensors are often crude. The size and position of the field of view, inparticular the position of the edges of the area of coverage, is notwell defined and may vary greatly from sensor to sensor. Similarly, thedetection threshold of the PIR sensor may vary from device to device andis generally either defined in manufacture or is user adjustable. In thecase of user defined detection threshold sensor, the user tends tosimply decrease alarm sensitivity after a false alarm. This user definedreduction in sensitivity is often to the point where the sensor wouldbarely detect an intruder.

Thermal detectors can also be manufactured as one or two dimensionalarrays of detecting elements. These detecting elements may beferroelectric, resistive, thermopile, diode or mechanical in operation.In the case of the ferroelectrics (which include pyroelectrics anddielectric bolometers) the output signal is transient, dying away evenif the scene information is constant. The resistive, diode, thermopileand mechanical devices have a constant response to a constant scenecontent.

Thermal imagers are imaging systems that use an infrared detector, whichmay be a thermal detector, to form images using the radiation emittednaturally by objects. This should be contrasted to PIR sensors which donot form images of the scene and hence their output contains no spatialinformation.

To date, testing of thermal detector modules after installation ishighly qualitative. The test typically consists of simply walking pastthe detector and observing the tell-tale trigger light that is typicallymounted on the detector module. This is termed the “walk-test”. Althoughthe walk test can provide an qualitative indication of whether thedetector module will be triggered by the specific person performing thetest, it is difficult to establish the exact boundaries of the detectorfield of view. This problem is exacerbated with pulse countingpyroelectric detector modules. In addition detection threshold levelsare often difficult, if not impossible, to determine. For example,unless a small animal is made to cross the field of view it can not bechecked whether such an event will cause the module to be triggered.Furthermore, the detector sensitivity may change over time or with driftof the electronics.

It should be noted that it is also possible to generate infra-redradiation using Light Emitting Diodes (LEDs). However, this energy isnot related to the device temperature. LEDs that emit blue, green andred light can be readily obtained, as can LEDs that emit at wavelengthsin the very near infra-red (VNIR) band between 0.8 μm and 1.0 μm. Theselatter LEDs are common in remote control devices for TV sets and carsecurity systems and their use in toys is described in EP0232157.

According to a first aspect of the invention, a device for communicatingwith a thermal detector module comprises means for providing acollimated beam of thermal infrared radiation; and means for modulatingthe intensity of the collimated beam of thermal infrared radiation,wherein the intensity modulated thermal infrared beam, when directed onto the thermal detector module from a position within the module's fieldof view, is suitable for triggering a response from the thermal detectormodule.

The term infrared radiation generally refers to radiated energy atwavelengths longer than that of visible light and shorter thanmillimetre waves. Within this spectrum, stretching from about 0.75 μm to1000 μm, there are a number of bands where the atmosphere istransparent.

The region from 0.75 μm to 1 μm is often known as the Very Near InfraRed(VNIR) and the region between 1 μm and 2.5 μm is often known as theShort Wave InfraRed (SWIR). VNIR and SWIR radiation is collectivelytermed near infrared radiation.

The primary bands for Thermal Imaging are from 3-5 μm (known as the ‘3-5band’, the Medium Wave InfraRed (MWIR) or ‘IR Band 2’) and from 8-14 μm(known as ‘the 8-14 band’, the Long Wave InfraRed (LWIR) or ‘IR Band3’). Herein, infra-red radiation having a wavelength greater than 3 μmbut less than 14 μm is termed thermal infra-red radiation.

The intensity modulated beam of thermal infrared radiation transmittedby the device will thus preferably have a wavelength within the 3 μm to14 μm wavelength range and will more preferably have a wavelength withinthe MWIR and/or LWIR band. Furthermore, the intensity modulated beam ofthermal infrared radiation will conveniently have a wavelength greaterthan 8 μm or greater than 10 μm.

It should be noted that it is the use of thermal infrared radiationwhich enables a response from the thermal detector module to betriggered. A source of near IR radiation, such as an infra-red LED ofthe type used in conventional infra-red remote controls for televisionsand the like, will not be suitable to trigger a response from thethermal detector module.

In accordance with the invention, it is thus possible to determine thefield of view of the thermal detector module in a quantitative manner.If the thermal detector module is user adjustable, it can be calibratedto be triggered by thermal events of a given magnitude that occur withina defined field of view. Alternatively, the performance criteria of athermal detector module can be verified. This provides a method ofcalibration or testing that is an improvement over the prior art walktest.

Advantageously, the means for intensity modulating the collimated beamof thermal infrared radiation comprises a chopper. The term chopper iswell known to those skilled in the art, and comprises a rotating bladehaving transparent and opaque segments that are sequentially placed inthe path of an optical beam. This causes intensity modulation of thebeam.

Alternatively, the means for intensity modulating the collimated beam ofthermal infrared radiation comprises a shutter. The shutter may beelectro-optic or mechanical.

Conveniently, the means for intensity modulating the collimated beamimparts a periodic intensity modulation to the thermal infrared beam.For example, a chopper may be provided that has a blade with equallysized and spaced segments of opaque and transparent materials. Periodicopening of shutter would also achieve the same effect.

A modulation frequency of around 1 Hz may advantageously be used. Amodulation frequency of this order of magnitude would trigger a responsefrom most commercially available pyroelectric detectors. A personskilled in the art would recognise that atypical thermal detectors mayrequire the use of a modulated beam having a lower or higher modulationfrequency. For example, a defined number of pulses may need to besupplied for pulse counting systems.

In a further embodiment, the thermal infrared beam can be modulated tocarry one or more digital codes. The inclusion of the digital codewithin the modulated infrared beam allows the response of anappropriately configured thermal detector to be remotely controlled.This is an advantage where the thermal detector is not readilyaccessible, or where the ability to alter the response of the detectorneeds to be restricted.

Advantageously, the device may additionally comprise a secondarytransmitter means suitable for sending control data to said thermaldetector module. For example, in addition to the intensity modulatedthermal infrared beam the device could comprises a near IR LED (e.g. 0.8μm ) that is appropriately modulated to carry information. The devicewould be used with a thermal detector module having a correspondingreceiver means (e.g. a photo-detector operating in the near IR) toreceive and interpret the control data transmitted by the secondarytransmitter means of the device. In this manner remote control of thefunctions of the thermal detector module would be possible.

Conveniently, the thermal infrared beam is collimated such that theoptical energy reaching the thermal detector module is substantiallyindependent of the distance of the device from the thermal detectormodule.

Advantageously, the means for providing a collimated beam of thermalinfrared radiation comprises a thermal infrared radiation source and oneor more infrared optical elements.

The one or more infrared optical elements may comprise one or moreinfrared lenses or one or more infrared reflective optical components. Acollimated beam may be provided by one or more lenses, and/or one ormore reflective optical components (e.g. focussing mirrors),appropriately arranged with respect to the thermal infrared radiationsource.

Advantageously, the thermal infrared radiation source is any one of aPeltier heat pump, an electrically heated component such as a wirefilament, a ceramic element (especially one having a positivetemperature coefficient) or a carbon rod. The power supply of theradiation source may be modulated and thereby provide the means formodulating the intensity of the collimated beam.

Conveniently, the temperature of the thermal infrared radiation sourceis controlled by a stabilisation circuit. Temperature stabilisation ofthermal infrared sources, which may include stabilisation of the powersupplied to the thermal infrared source, is well known to those skilledin the art.

In a further embodiment, means are provide to vary the intensity of thethermal infrared beam. For example, a plurality of infrared filterscould be provided to reduce the power of the infrared beam withoutaltering the temperature (i.e. wavelength) emitted by the infraredsource.

In a further embodiment, the device also comprises a collimated visiblebeam of light that is emitted along substantially the same optical pathas the collimated beam of thermal infrared radiation. The collimatedvisible beam of light may be provided by a laser source. This allows theuser of such a device to direct the thermal infrared beam (which is notvisible to the human eye) to a particular point, such as to the thermaldetector.

According to a second aspect of the invention, a thermal detector modulecomprises a thermal detector and electronic circuitry that is programmedto analyse detected thermal infrared radiation and output an electricalresponse when triggered by a certain thermal event, characterised inthat the certain thermal event which causes the thermal detector moduleto output an electrical response can be varied by re-programming theelectronic circuitry using digital codes received by the thermaldetector in a modulated thermal infrared beam.

Such thermal detector modules provide significant advantages over knownthermal detector modules where any reprogramming is performed usingcontrols provided on the detector module or any central control unit forthat module.

Advantageously, the thermal detector may be a single elementpyroelectric detector, a pyroelectric imaging array or a thermal imagingarray.

Conveniently, the thermal detector comprises a single element.Alternatively, the thermal detector comprises a plurality of elements.The element or elements may be of the pyroelectric type, the resistancebolometer type or another suitable type.

Conveniently, the active field of view of a pyroelectric imaging arraymodule or a thermal imaging array module can be re-programmed usingdigital codes received by the module in a modulated infrared beam.

Advantageously, the threshold sensitivity of the thermal detector modulecan be re-programmed using digital codes received by the thermaldetector module in a modulated thermal infrared beam.

According to a third aspect of the invention, a thermal detector kitcomprises a thermal detector according to the second aspect of theinvention and a device according to the first aspect of the invention.

According to a fourth aspect of the invention, a method of testing thefunction of a thermal detector module comprising the steps of (a) takinga radiation source that emits a collimated and intensity modulated beamof thermal infrared radiation, (b) directing the beam of thermalinfrared radiation onto the thermal detector module from within thefield of view of that thermal detector, and (c) observing whether thethermal detector module is triggered by the beam of thermal infraredradiation. This permits the functionality of the thermal detector moduleto be quickly assessed.

According to a fifth aspect of the invention, a method of establishingthe field of view of a thermal detector module comprising the steps of(a) taking a radiation source that emits a collimated and intensitymodulated beam of thermal infrared radiation, (b) directing the beam ofthermal infrared onto a thermal detector module from a plurality ofdifferent positions, and (c) determining from which positions thethermal detector module is triggered thereby allowing the field of viewof the thermal detector module to be established.

Conveniently, the thermal detector module comprises one or more elementswhich may be of the pyroelectric type, the resistance bolometer type oranother suitable type and the method may comprise the additional step ofaltering the field of view or sensitivity of the thermal detector modulesuch that the desired field of view properties are obtained from thethermal detector module.

Advantageously, the thermal infrared beam is modulated to carry adigital code and the field of view or sensitivity of the thermaldetector module is altered in response to the digital code it receives.

According to a sixth aspect of the invention, a method of determiningthe threshold sensitivity of a thermal detector module comprises thesteps of (a) taking a radiation source that emits a collimated andintensity modulated beam of thermal infrared radiation, (b) directingthe beam of thermal infrared radiation onto a thermal detector modulefrom within the field of view of that thermal detector module, and (c)altering the intensity of the thermal infrared beam so that thethreshold sensitivity of the thermal detector module can be established.

The invention will now be described, by way of example only, withreference to the following figures in which;

FIG. 1 shows a thermal detector calibration device;

FIG. 2 shows a thermal detector calibration torch with an integratedvisible laser pointing beam; and

FIG. 3 shows a thermal detector calibration device for communicatingwith a suitably adapted thermal detector module.

Referring to FIG. 1, a calibration device 2 for a thermal detectormodule is shown. The calibration device comprises a thermal infra-redradiation source 4, a collimating lens 6 and a chopper 8. The chopper 8comprises a motor 10 that is connected via a spindle 12 to a blade 14.The blade 14 has infrared transparent 16 and infrared opaque 18portions.

In operation, thermal infrared radiation from the radiation source 4 iscollimated by the lens 6 and is modulated by the chopper 8 at afrequency of around 1 Hz such that a collimated and modulated beam ofthermal infrared radiation 20 is produced. The precise modulationfrequency of the radiation is determined by the frequency to which thethermal detector module will respond to. For typical thermal detectors,a frequency of around 1 Hz can be used although the exact frequency isnot critical.

The thermal infra-red radiation source 4 may have a heated element thatcomprises a Peltier heat-pump, an electrically heated component such asa wire filament, a ceramic element (especially one having a positivetemperature coefficient) or a carbon rod. To ensure a constanttemperature, the heating element is supplied with electrical currentunder the control of a temperature stabilisation circuit. Again,techniques for accurate temperature control are well known to thoseskilled in the art.

It should be noted that a blackbody radiator radiates energy with anintensity that is determined only by its temperature and dimensions. Itis not possible to make a true black-body, although 99% of thetheoretical performance can be achieved with care. However, although asource may have only 80% of the black-body performance it is thestability of the performance that is important. The dimensions of thebody are fixed and therefore stable, which leaves only the temperatureof the body to be controlled. As described above, such temperaturecontrol is well known to those skilled in the art.

The intensity of the modulated beam of thermal infrared radiation 20 maybe reduced by the insertion of a controllable aperture or a filter intothe optical path. This permits the intensity of the infra-red radiationto be reduced without altering the temperature of the element in theradiation source 4.

In place of, or in addition to, the collimating lens 6 additionaloptical components (not shown) may be provided. For example, thecollimating function may be provided by one or more suitably adaptedfocussing mirrors in place of the lens 6.

In place of the chopper 8, alternative infra-red radiation modulationmeans (not shown) may be provided. For example, a mechanical orelectro-optic shutter could be provided. The shutter could provide abeam of infra-red radiation with an information carrying modulatedsignal.

Referring to FIG. 2, a calibration torch device 30 is shown. Likecomponents to those shown in FIG. 1 are given like reference numerals.

The calibration torch device houses a calibration device 2 as describedwith reference to FIG. 1 that provides a modulated beam of thermalinfrared radiation 20. In addition to the thermal infrared radiation,the calibration torch emits a visible laser beam 32. Production of sucha laser beam 32 is well known to those skilled in the art. This visiblelaser beam 32 is directed along substantially the same path as themodulated beam of thermal infrared radiation 20. In this way it ispossible for a user to direct the modulated beam of thermal infra-redradiation to a specific point in space using the visible laser beam 32as a guide. Collectively, the thermal infrared beam 20 and the visiblelaser beam 32 are termed the torch output beam 36.

Referring to FIG. 3, a thermal detector module system 38 is shown. Likecomponents to those shown in the previous figures are given likereference numerals.

The thermal detector module system 38 comprises a calibration torchdevice 30 and a thermal detector module 40. The thermal detector module40 comprises an outer window 42 which also acts as an infrared mirror orlens to direct incident radiation on to an infrared detector 44. Theinfrared detector 44 may comprise one or more discrete detectionelements (e.g. it may be a multi-segment detector or a multi-pixeldetector array). The output of the infrared detector 44 is analysed bysuitable detection electronics, which provide a trigger response when acertain predetermined IR event is detected.

To perform a calibration, the torch output beam is directed onto theouter window 42 of the thermal detector module 40. In the case of athermal detector that is triggered by transient events, the user of thecalibration torch should remain substantially stationery during eachtest so that his or her thermal signature does not itself trigger thesensor.

The sensitivity and coverage of the thermal detector module can thus bedetermined by directing the beam 20 onto the outer window 42 of thethermal detector module 40 from multiple different positions, andpossibly using beams of different optical power. As the power reachingeach discrete detection element is known, the response of the detectorto such an infrared signal can be assessed. In other words, anadjustable thermal detector module can be calibrated such that it istriggered by a certain level of infra-red radiation incident upon itfrom a defined field of view. If a thermal detector module has factoryfixed setting, the calibration process can be used to determine if thedetector still meets the necessary performance specifications.

Even if quantitative calibration is not required, the calibration torchmay be used to very quickly test the functionality of any thermaldetector module. For example, the calibration torch may be used todetect any sabotage involving coating the detector optics with infraredopaque substances, or if the sensor has failed.

In a further development, the calibration torch device may also be usedto send data to the thermal detector module 40 in the form of a digitalcode. For example, different gaps could be provided in the chopper blade14 or a shutter could be used to provide a more complex modulated IRbeam.

In this case, the thermal detector module 40 obviously has to be adaptedso as to respond appropriately to any data it receives. For example, ifa pyroelectric imager array or thermal imager was used it could beprovided with software which recognises the digital code and performs acertain act (e.g. activates a test circuit, resets or recalibrates)accordingly. An intruder thermal detector module could also provide avisual confirmation of the receipt of the digital data, for example bydouble blinking its LED.

It would also be possible for the calibration torch to send, and thethermal detector module to receive, a digital code that changes the modeof operation of the thermal detector module. For example, the digitalcode could be used to adjust detector sensitivity, upload software,label zones within an art gallery, label the edges of a garden, drivewayor swimming pool, or to define the entry and exit points in a room.Alternatively, or additionally, the torch could also comprise a separatetransmitter (e.g. a RF transmitter or an IR LED) to send a controlsignal to the thermal detector module.

1. A thermal infrared detector module, wherein thermal infraredradiation has a wavelength range of from 3-14μm, said module comprising:a thermal infrared radiation detector, said detector having a field ofview; and electronic circuitry, responsive to said detector analysingdetected thermal infrared radiation and for outputting an electricalresponse when triggered by a certain thermal event within said field ofview, wherein said certain thermal event to which said electroniccircuitry outputs a response is variable by re-programming theelectronic circuitry by means of a digital code in a modulated thermalinfrared beam received by the thermal infrared detector.
 2. A module asclaimed in claim 1 wherein the thermal infrared detector comprises asingle element.
 3. A module as claimed in claim 1 wherein the thermalinfrared detector comprises two or more elements.
 4. A module as claimedin claim 3 wherein the thermal infrared detector comprises a twodimensional array of elements.
 5. A module as claimed in claim 1 whereinthe thermal infrared detector comprises pyroelectric or resistancebolometer elements.
 6. A module as claimed in claim 1 wherein the activefield of view of the thermal infrared detector module is re-programmableusing digital codes received by the thermal infrared detector module insaid modulated thermal infrared beam.
 7. A module as claimed in claim 1wherein the threshold sensitivity of the thermal infrared detectormodule is re-programmable using digital codes received by the thermalinfrared detector module in said modulated thermal infrared beam.
 8. Athermal infrared detector kit comprising a module as claimed in claim 1and a device for producing an intensity modulated thermal infrared beam.9. A method of testing the function of a thermal infrared detectormodule according to claim 1 comprising the steps of: a) providing aradiation source that emits a collimated and intensity modulated beam ofthermal infrared radiation; b) directing the beam of thermal infraredradiation onto the thermal infrared detector module; and c) observingwhether the thermal infrared detector module is triggered by the beam ofthermal infrared radiation.
 10. A method according to claim 9 whereinthe step of (b) directing the beam of thermal infrared radiation ontothe thermal infrared detector module comprises directing the beam ofthermal infrared radiation onto the thermal infrared detector modulefrom a plurality of different positions and the step of (c) observingwhether the thermal infrared detector module is triggered by the beam ofthermal infrared radiation comprises the step of establishing the fieldof view of the thermal infrared detector module.
 11. A method as claimedclaim 9 wherein the field of view or sensitivity of the thermal infrareddetector module is altered in response to the digital code it receives.12. A method according to claim 9 wherein the step of (b) directing thebeam of thermal infrared radiation onto the thermal infrared detectormodule comprises directing the beam of thermal infrared radiation ontothe thermal infrared detector module from within the field of view ofthat thermal infrared detector module and the step of (c) observingwhether the thermal infrared detector module is triggered by the beam ofthermal infrared radiation comprises the step of altering the intensityof the thermal infrared beam so that the threshold sensitivity of thethermal infrared detector module can be established.
 13. A module asclaimed in claim 1 wherein the thermal infrared radiation wavelength isin a sub-band of from 3-5μm.
 14. A module as claimed in claim 1 whereinthe thermal infrared radiation wavelength is in a sub-band of from8-14μm.
 15. A device for communicating with a thermal infrared detectormodule, wherein thermal infrared radiation has a wavelength range offrom 3-14μm, said device comprising: an infrared source for providing acollimated beam of thermal infrared radiation; and a modulator formodulating the intensity of the collimated beam of thermal infraredradiation, wherein the intensity modulated thermal infrared beam, whendirected on to the thermal infrared detector module from a positionwithin the module's field of view and triggers reprogramming of thethermal infrared detector module.
 16. A device according to claim 15 andfurther comprising a collimated visible light source, the collimatedvisible light source being arranged to produce a beam of visible lightthat is emitted along substantially the same optical path as thecollimated beam of thermal infrared radiation.
 17. A device according toclaim 15 wherein the modulator is arranged to modulate the thermalinfrared beam to carry one or more digital codes.
 18. A device accordingto claim 15, wherein the thermal infrared radiation wavelength is in asub-band of from 3-5μm.
 19. A device according to claim 15, wherein thethermal infrared radiation wavelength is in a sub-band of from 8-14μm.